This invention relates to an apparatus for incinerating combustible material and in particular to a rotary kiln apparatus for incinerating waste materials such as municipal solid waste or commercial waste materials.
Prior to the present invention, it was known to incinerate municipal, commercial solid waste material and hazardous materials to achieve volume reduction and/or eliminate hazardous elements before being placed in a landfill. Typically, the solid waste includes approximately twenty-five percent non-combustibles (ash), thirty percent moisture, typical ranges are between fifteen percent and seventy-five percent moisture, and forty-five percent combustibles. The higher the percentage of moisture that exists in the waste, the more difficult it is to achieve proper ignition and complete combustion of the combustible materials.
In order to achieve low operating costs, a solid waste incineration system must be designed to utilize a minimum of auxiliary fuel. Most of the heat requirement should come from the heating value in the solid waste.
One method for solid waste incineration is mass burning of the material on a stoker/grate boiler. While this technology is proven and well established, the capital cost for such facilities are high.
Prior to the present invention, rotary kilns have been utilized as a alternate to stoker grate boilers, as they require less capital cost and can be economically utilized for smaller quantities of wasted material. One of the disadvantages of the rotary kiln is that when the material flow in the kiln is counter-current to the gas flow, the gas exit temperature is lowered by the cold, wet feed prior to the gas leaving the kiln. Since the exhaust gases may contain some combustible material or carbon monoxide, extra fuel must be added to an afterburner to complete the combustion of the combustibles in the gas stream.
To burn the solid waste in a rotary kiln in a co-current gas-material or parallel flow configuration, in order to more easily start ignition of the bed of materials, keep exit gas temperatures high, and to keep the use of afterburner auxiliary fuel to a minimum, it is necessary to predry the material. According to present technology, a separate drier is utilized ahead of the kiln to dry the material. This drier is usually a grate drier or rotary drier. This additional piece of equipment requires additional capital outlay and increases installation and operating costs.
An alternate process utilizes mass burning in a water walled waste incineration boiler and associated equipment tend to provide a reduced temperature burning zone because of the cooling effect of the water walls thus encouraging the formation of undesirable combustion byproducts.
According to the present invention, a device has been provided which allows the drying of the solid material to occur inside the rotary kiln in a co-current gas-material flow system. While rotary kilns using co-current or parallel gas and material flow are known as for example in U.S. Pat. No. 3,436,061 issued Apr. 1, 1969, the present invention provides a novel arrangement for achieving predrying and co-current flow of the combustion gases and burning wastes to facilitate high operating temperatures in the range of 1600.degree. F. to 3200.degree. F. in the combustion and discharge zones. These high burning temperatures assisted by radiant reflected heat from the refractory lined kiln together with efficient air supply helps to produce complete combustion and therefore avoids the formation of undesirable by-products such as dioxins, carbon monoxide, combustible particulate material and similar gaseous and particulate emissions in the exhaust gases and ash streams.